Communication terminals transfer data to and from other communication terminals along a communication channel, such as a telephone network. Often, the communication terminals are digital apparatus, such as digital computers or facsimile machines, which transfer data in digital form. Typically, a digital communication terminal connects to a telephone network by means of a telephone communication interface apparatus such as a modem, i.e., a modulator-demodulator. In transferring digital data, the goal is .to transfer data quickly and accurately.
In general, transferring digital data requires that the digital communication terminal sending the digital data first organize the digital data according to a data format and then send the formatted data to a sending interface apparatus. The sending interface apparatus processes, or modulates, the formatted digital data into a signal that can be sent along the telephone network. The telephone network carries the modulated data signal to a receiving interface apparatus that demodulates it into the formatted digital data and sends the formatted digital data to the receiving digital communication terminal. Ideally, the receiving interface apparatus receives the digital data in minimum time and with minimum errors.
The format of digital data typically includes sequences, or blocks, of characters that represent information, i.e., data, in digital form. Each block of characters also includes overhead characters that tell the receiving communication terminal when a particular block it is receiving begins and ends. Certain other characters, which are useful in detecting errors in the block, such as wrong, missing or extra characters, that may be caused during the transfer of the block between communication terminals, are generally included as well. Error correction, however, is limited for the most part to detecting that a block contains an error without determining what particular characters are wrong, missing, or extra.
Errors in a character block received by the receiving interface apparatus typically are caused by some form of noise interfering with the receiver's reception of the modulated data signal. This occurs when the receiver is exposed to noise having a power level that is significant compared to the power level of the received modulated data signal. What constitutes a significant ratio of signal-to-noise depends on numerous factors. Generally, the lower the signal-to-noise ratio, the greater the rate of errors.
The length of a block of characters can range from one character to thousands of characters, and different blocks may vary in length. However, because of the large number of overhead characters normally associated with each block, each block is preferably as large as possible to increase throughput, i.e., the total number of data characters, not including overhead characters, received per unit of time. Since a block must be retransmitted if it contains a detected error, increasing the size of a block has the drawback of increasing the chance that for a given error rate a block will contain a detected error as well as the drawback of increasing the time required to retransmit a block. Consequently, a compromise block size is generally chosen, typically between 128 and 1024 characters for a telephone communication interface apparatus operating at a data transfer rate of 1200 baud.
One common type of telephone communication interface apparatus used to receive data is an acoustic interface apparatus, which is connected, or coupled, to the telephone network acoustically. A telephone speaker produces sound or acoustic signals that correspond to the modulated digital data carried by the telephone network, and an acoustic interface apparatus receives such sound signals by means of a microphone placed near the telephone speaker. The microphone is usually mounted inside a muffling cup which engages the earpiece of the telephone handset and which serves to reduce the level of ambient noise received by the microphone from its surroundings.
Since the microphone is usually omni-directional and the material from which the muffling cup is made does not tend to be a good absorber of sound or vibration, the microphone may receive ambient noise having significant power compared to the power of the received sound signal despite the use of a muffling cup. Consequently, the signal-to-noise ratio may be low enough to cause a significant number of errors in the blocks of characters received. Although the resulting error rate can require frequent retransmittals of character blocks, it generally is not high enough that reception of the digital data is prevented entirely.
Ambient noise typically interferes little with another type of telephone communication interface apparatus, known as an inductive interface apparatus, which does not use acoustic coupling. An inductive interface apparatus connects or couples the telephone network to the receiving communication terminal inductively. A telephone speaker produces an electromagnetic signal corresponding to the modulated data signal carried by the telephone network, and an inductive interface apparatus receives such electromagnetic signals by means of a suitable electromagnetic transducer, such as an inductive pickup coil, placed near the telephone speaker.
Similar to acoustic coupling, inductive coupling is subject to errors that result from the electromagnetic transducer receiving electromagnetic noise having a power level which is significant compared to that of the received electromagnetic signal. Electromagnetic noise sources include, for example, electric motors and switching power supplies. With inductive coupling, however, the primary concern is with electromagnetic signal power, not audible noise power. Although the speakers of all telephones produce electromagnetic signals, different types of telephones tend to produce electromagnetic signals having different powers. Some telephones produce electromagnetic signals having powers that are relatively low compared to typical electro-magnetic noise power. Consequently, the received signal-to-noise ratio is low enough that the error rate becomes significant. Other telephones produce electromagnetic signals having high power compared to typical noise power and, consequently, the error rate is insignificant. For example, public access phones are required by federal law to produce electromagnetic signals of sufficient power to be received by a hearing aid equipped with an appropriate electromagnetic transducer. Telephones for in-home or business use, on the other hand, are not required to meet this requirement.
Another type of telephone communication interface apparatus provides both acoustic and inductive modes. The choice of which mode to use is made manually at the start of the receipt of the digital data. However, this is a disadvantage because there is no determination of which mode can receive the data in the shorter time and with fewer errors.
There is, therefore, a need for an interface apparatus which receives data in either the acoustic mode or the inductive mode, but which avoids the disadvantages of manual selection. The present invention fulfills this need.